coding the back end order management for day trading is not that difficult? However that is where most who try to automate trading fail
Not sure what you mean by 'most' - plenty of firms do it everyday. If you mean a one man/woman band creating their own code, and with limited cash AND unable to sustain some losses along the learning curve, sure, its impossible.
Which is why I suggested earlier to read VIRT's IPO, and other disclosures on how they operate to understand the infrastructure required, before thinking about trying it. As always, the Devil is in the details, such as, their credit lines, their staff , trading levels etc and the other article which describes their road to success.